Hi --- If something is wrongly marked with a lower price, you cannot insist on buying it at that price. When a retailer displays a product for sale, legally it is giving you 'an invitation to treat', which means it is inviting you to make an offer to buy.
The retailer can refuse that offer if it decides that it doesn't want to sell you the goods, to have a legally-binding contract the retailer must have accepted your offer to buy.
The law says that a retailer can refuse to sell anything to you, at any price, without giving a reason. But if a retailer sells you something at too low a price by mistake, they can’t later make you pay the extra, unless they can show you knew the price was wrong.
Under the Consumer Protection Act 1987, it is illegal to give misleading price information about goods and services, however their innocent explanation in this case that the offer had just finished and they were in the process of updating their prices, would suffice.